Radial polarization-rotating optical arrangement and microlithographic projection exposure system incorporating said arrangement

ABSTRACT

An optical arrangement is disclosed wherein an entering beam is converted into an exiting beam having a total cross section of light which is linearly polarized essentially in the radial direction by rotation. For this purpose, rasters of half-wave plates ( 41, 42, 4 i), a combination of birefringent quarter-wave plate  420  and a circular plate  430  is suggested in combination with a conical polarizer  21 ′. This arrangement is preferably utilized in the illumination portion of a microlithographic projection exposure system. It is important that the arrangement be mounted behind all asymmetric or polarizing component elements  103   a.

This is a divisional of patent application Ser. No. 09/352,408, filed onJul. 14, 1999, now U.S. Pat. No. 6,191,880, which is a continuation ofapplication Ser. No. 08/717,902, filed on Sep. 23, 1996, now abandoned.

FIELD OF THE INVENTION

The invention relates to an optical arrangement which converts anentering light beam into an exiting light beam having light which islinearly polarized in the entire cross section essentially in radialdirection.

BACKGROUND OF THE INVENTION

It is necessary to provide projection exposure systems with a very highnumerical aperture in order to achieve the highest resolutions inmicrolithography. Light is coupled into the resist layer at very largeangles. When this light is coupled in, the following occur: light lossesbecause of reflection at the outer resist boundary layer anddeterioration of the resolution because of lateral migration caused byreflections at the two boundary layers of the resist to the wafer and tothe air (formation of standing waves).

The degree of fresnel reflection is then dependent upon the anglebetween the polarization direction and the reflection plane. Thereflection vanishes when light having an electrical field oscillatingparallel to the incident angle incidents at the brewster angle. Thisprovides for optimal in-coupling into the resist while at the same timeproviding maximum suppression of the standing waves.

However, disturbances occur for light which is linearly polarized in onedirection as described in European patent publications 0,602,923 and0,608,572. Accordingly, the apparatus disclosed in these publicationsgenerate circularly polarized light which is coupled into the resist asthe equivalent of unpolarized light. In this way, homogeneity isachieved over the entire image. However, a loss of efficiency isaccepted because in each case, the locally perpendicular polarized lightcomponent is intensely reflected.

In European patent publication 0,602,923, it is alternatively suggestedthat linearly polarized light should be orientated in one directionrelative to the orientation of a pattern to be imaged as alreadydisclosed in German patent publication 1,572,195. The penetration via amultiple reflection takes place in the longitudinal direction of thestructures and not in the direction of the critical resolution. Theefficiency of the in-coupling or the reflection at the resist surface ishowever not homogeneous.

The effect of the polarization on the reflection at the resist layersand the significance of the fresnel coefficients is described in U.S.Pat. No. 4,899,055 directed to a method for measuring thickness of thinfilms.

U.S. Pat. No. 5,365,371 discloses a projection exposure apparatus formicrolithography wherein a radially directed linear polarization of thelight is introduced in order to prevent disturbances because of standingwaves in the resist when generating images therein. Two differentpolarization elements are given, namely, a radial polarization filtercomposed of a positive cone and a negative cone. This filter is utilizedin transmission and effects radial polarization for the reflectionbecause of the fresnel equations. However, it is not disclosed how acomplete polarization of the transmitted light is achieved. In thedescription of U.S. Pat. No. 5,365,371 and in claim 3 thereof, it isrequired in addition that both parts have different refractive indices.The transmitted part must then however be deflected and cannot pass in astraight line.

U.S. Pat. No. 5,436,761 has a disclosure identical to that of U.S. Pat.No. 5,365,371 referred to above and includes a single claim wherein nocondition is given for the indices of refraction. Furthermore, in claim4 of U.S. Pat. No. 5,365,371, a plate having segments of radiallyorientated polarization filter foils is given as is known from U.S. Pat.No. 4,286,843 (see FIG. 19 and column 9, lines 60 to 68).

Both polarizers are polarization filters, that is, they lead to highlight loss and are suitable only for an incoming light beam which isunpolarized or circularly polarized because, otherwise, an intensenonhomogeneity of the intensity would occur over the cross section ofthe exiting light beam.

In the example of FIG. 1 of U.S. Pat. No. 5,365,371, the deflectingmirror 17 causes a partial polarization and therefore the light beamexiting from the polarizer 21 is nonhomogeneous.

U.S. Pat. No. 5,365,371 discloses that the radial polarizer lies in thepupillary plane of the projection objective. A position of the radialpolarizer in the objective is problematical because there, the tightesttolerances for an optimal image quality must be maintained.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an optical arrangement whichpermits a homogeneous coupling of light into optical boundary surfaceswith high aperture and with low loss and low scattered light. It isanother object of the invention to provide such an arrangement whereinthe efficiency and the homogeneity of the exiting light beam areoptimized.

Projection exposure apparatus are provided which permit maximum use ofthe advantages of radial linear polarization with minimum disturbance ofthe imaging and minimum complexity with respect to assembly.

The invention is directed to an optical arrangement which includes: anoptical structure for receiving an entering light beam; the enteringlight beam having a linear polarization (P) in a predetermineddirection; and, the optical structure being adapted to convert theentering light beam into an exiting light beam wherein the direction ofthe linear polarization (P) is, however, not subtractively selected butis instead rotated essentially over the entire cross section of theexiting light beam.

In this connection, it is noted that normal polarizers effect aselection. Thus, a polarization direction is permitted to pass and theorthogonals are, for example, removed from the light beam by reflection,refraction and absorption. Accordingly, unpolarized light yields amaximum of 50% linear polarized light. When linear polarized lightenters a polarizer at an angle to the direction of polarization, theprojection of the polarization vector is selected to the polarizationdirection for through passage and the orthogonals are eliminated. Incontrast, in the optical arrangement of the invention, the direction ofthe linear polarization is actually rotated.

Advantageous embodiments are disclosed which provide different ways ofgenerating the desired polarization distribution. One embodimentincludes ring aperture illumination wherein the incident light at lowangles (for which low angles the reflectivity is only slightly dependentupon polarization) is suppressed.

Another embodiment is directed to the integration of a radiallypolarizing optical arrangement into a microlithographic projectionexposure system.

In this system, the possibilities of the optics are fully utilized andan improvement in the homogeneity and in the efficiency of couplinglight into the resist layer is achieved because the reflection at theresist layer is reduced uniformly. However, uniform reduction is alsoachieved at all lenses arranged downstream of the polarizing element.For the light incident at large angles (up to the brewster angle), theeffect is the greatest especially where the light intensity (peripheraldecay) is at the lowest. The disturbances of the resolution because ofscattered light, even at the resist wafer boundary layer, arehomogenized and reduced.

An arrangement close to start of the beam path is advantageous becausethe disturbances caused by stress-induced birefringence at alldownstream lenses is minimized and made symmetrical.

For this reason, it is also advantageous for polarization filters (inaddition to the preferred polarization-rotating elements) when theseelements are mounted in the illuminating system.

In another embodiment, the polarization-rotating elements are mounted atany desired location in a projection illuminating system which ischaracterized by improved homogeneity and a much higher efficiencycompared to the state of the art.

In another embodiment, a reduction and homogenization of the scatteredlight occurs at each lens of the system (even with a low angle ofincidence).

On the other hand, asymmetrical optical elements change the state ofpolarization and can therefore only be arranged downstream when areflecting layer having phase correction is utilized. This is especiallythe case for deflecting mirrors such as for shortening the structurallength or as provided in catadioptric projection objectives. If atotally-reflecting prism is utilized as a deflecting element, then aprecisely adapted phase-retarding plate must be mounted downstream orthe totally reflecting boundary layer must be coated with aphase-correcting layer. Polarizing optical elements such as polarizationbeam splitters and quarter-wave plates are also disturbing.

The invention is also directed to a microlithographic projectionexposure system incorporating a radially polarizing optical arrangement.More specifically, the microlithographic projection exposure system ofthe invention includes: a light source defining an optical axis andtransmitting a light beam along the optical axis; an optical structurearranged on the optical axis for receiving the light beam; the enteringlight beam having a linear polarization (P) in a predetermineddirection; and, the optical structure being adapted to convert theentering light beam into an exiting light beam wherein the direction ofthe linear polarization (P) is rotated essentially over the entire crosssection of the exiting light beam.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawingswherein:

FIG. 1a is a plan view of a radially polarization-rotating opticalarrangement of a raster of half-wave plates for linearly polarizedincident light;

FIG. 1b shows the polarization directions of the light beam exiting fromthe arrangement of FIG. 1a;

FIG. 2 is an elevation view, in section, of a radially polarizingoptical arrangement having a conical-frustrum reflector having abrewster angle for circularly polarized or non-polarized incident light;

FIG. 3a is an arrangement incorporating a conical-frustrum reflector andsegmented half-wave plates for complete utilization ofcircularly-polarized light or non-polarized light;

FIG. 3b is a plan view of the arrangement of FIG. 3a as viewed from thelight exit end thereof;

FIG. 4a is a radial polarization-rotating optical arrangement having aplate with a central-symmetrical stress-induced birefringence;

FIG. 4b is a plan view of the quarter-wave plate of the arrangement ofFIG. 4a;

FIG. 4c is a plan view of the compressive-strain plate of thearrangement of FIG. 4a;

FIG. 4d is a plan view of the birefringent 45° plate corresponding tothe arrangement of FIG. 4a;

FIG. 5 is a schematic representation of a microlithographic projectionexposure system incorporating a radially polarizing optical arrangementin the illumination portion thereof;

FIG. 6 is a schematic representation of a catadioptric projectionobjective having a radially polarizing optical arrangement of theinvention incorporated therein; and,

FIG. 7 is a schematic representation of a microlithographic projectionexposure system incorporating the radially polarization-rotating opticalarrangement of a half raster of wave plates as shown in FIGS. 1a and 1b.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

A polarization-rotating arrangement according to the invention is shownin FIGS. 1a and 1 b as it is suitable especially in combination with ahoneycomb condenser for the conversion of linearly polarized light. Thisarrangement is especially suited for lasers as a light source. The beamcross section is subdivided into a multiplicity of facets (11, 12, 1 i)which is, in each case, made of a half-wave plate of birefringentmaterial. Each facet 1 i corresponds to a honeycomb element of thehoneycomb condenser. The facets 1 i are preferably cemented to thehoneycomb or placed in wringing contact therewith. For extreme radiationloads, the facets can be separately held and coated to preventreflection. The honeycomb condensers conventional for microlithographicprojection exposure systems have about 10² honeycomb elements and thenumber of the facets is the same.

The main axes (21, 22, 2 i) of the facets (11, 1 i) are each aligned inthe direction of the angle bisector between the polarization directionof the entering linearly polarized light and the radius (which isaligned to the particular optical axis A of the light beam and of thehoneycomb condenser) through the center of each facet 1 i. In this way,each half-wave plate facet 1 i effects the rotation of the polarizationdirection in the direction of the above-mentioned radius. FIG. 1b showsthis effect. Here, the entry surfaces (41, 42, 4i) of the honeycombcondenser are shown with the polarization directions (31, 32, 3 i) ofthe particular component beams which are all aligned radially.

The raster with hexagonal facets 1 i is only one embodiment which isespecially adapted for the combination with a honeycomb condenser. Otherrasters and especially fan-shaped sector subdivisions of the half-waveplates (see FIG. 3b) are also possible. The number of the individualelements can then be in the area of 10¹.

A reduction of the total degree of reflection at an optical boundarysurface compared to unpolarized light takes place so long as thecomponent of the light, which is polarized perpendicularly to the planeof incidence, is less than the component of the parallelly polarizedlight. This boundary case is achieved with only four 90° sectors havinghalf-wave plates so that, preferably, more half-wave plates are arrangedin the cross section of the light beam especially in the order ofmagnitude of 10 to 10² facets or sectors.

In contrast to the known radial polarizers with sectors as shown in U.S.Pat. Nos. 4,286,843 and 5,365,371, the polarization is filtered out withan insignificant amount of loss; instead, the light is changed atminimal loss in its polarization direction via birefringent elements.

The arrangement shown in FIG. 2 effects a continuous radial direction ofthe linear polarization for entering unpolarized or circularly polarizedlight 40. This arrangement is a polarization filter and is basicallyknown from U.S. Pat. No. 5,365,371 but is new with respect to itsdetails.

The conical frustrum 20 has a through bore and is made of a transparentmaterial, such as glass FK5, quartz glass or CaF₂, with the conicalangle α corresponding to the brewster angle and a dielectric reflectioncoating on the conical surface 21. The component 45 of the light beam 40is polarized perpendicularly to the incident plane and is thereforecompletely reflected. The transmitted beam 4 p is polarized completelyparallel to the incident plane and is therefore everywhere linearlypolarized radially to the optical axis A. The conical frustrum 20 isadapted for an annular aperture illumination and ensures the shorteststructural length. A complete cone is also suitable. The conicalfrustrum 20 is supplemented by a suitable hollow cone 22 to form acylinder ring whereby the reflective conical surface 21 is protected andthe entire structure is easier to mount. The conical frustrum 20 and thehollow cone 22 have the same index of refraction so that the lightpassing therethrough does so without refraction at the conical surface21, which is in contrast to U.S. Pat. No. 5,365,371.

FIG. 3a shows, in section, a further embodiment of that shown in FIG. 2wherein the reflective component 4 s is also utilized so that anarrangement with a substantially lower than 50% light loss is achievedbecause the polarization is effectively rotated and not filtered.

A transparent part 30 having a conical surface 31 is mounted about theconical frustrum 20′ having the conical surface 21′ corresponding toFIG. 2 (with an extending cylindrical extension portion). Thetransparent part 30 has a reflective cone surface 31 parallel to theconical surface 21′. A ring of segments (5 i, 5 k) of half-wave platesis mounted on the exit surface 32 of the part 30. The main axes (6 i, 6k) of the segments are at 45° to the radius in the segment center asshown in FIG. 3b. In this way, and as described with respect to FIG. 1,the radial linear polarization is effected also of the light 4 sreflected at the conical surface 21′ in the beam 4 r parallel to theaxis. The effected increase of the light-conductance value is oftendesired at least for laser light sources. It is important that thearrangement is suitable for unpolarized incident light. By omitting oradding optical glass, the optical path of conical frustrum 20′ andtransparent part 30 can be adapted.

An arrangement for continuously generating radially linear polarizedlight is shown in FIGS. 4a to 4 d. Here, the arrangement is for linearlyor circularly polarized light at the input with reduced structurallength in the direction of the optical axis. It is especially suitablefor annular aperture optics.

An annular beam of uniformly linear polarized light 41 impinges on astack of three planar plates (410, 420, 430) as shown in section in FIG.4a. Planar plate 410 is a quarter-wave plate which, as FIG. 4b shows,circularly polarizes the through-passing light. If the entering beam isalready circularly polarized, then the plate 410 can be omitted. A plate420 follows and can, for example, be made of glass or quartz glass. Theplate 420 is under centrally-symmetrical pressure stress and hastherefore stress-induced birefringence. Thickness, material and stressare so selected that the plate 420 is a quarter-wave plate in the outerregion touched by the annular beam 41 but with radial symmetry so thatthe circularly polarized entering light is linearly polarized and withthe polarization direction at 45° to the radius over the entire crosssection as shown in FIG. 4c.

Such a pressure stress always accompanies thermal expansion andtemperature gradients when cooling or a compensating thermal treatmentin circularly round glass plates (or plates of quartz glass,berylliumfluoride, CaF₂ et cetera) . The pressure stress is normallyminimized with the longest possible cooling. Via deliberate cooling, thedesired pressure stress can be generated within wide limits andtherefore the desired stress-induced birefringence is generated in theexterior region.

A third plate 430 follows which has circular birefringence and rotatesthe polarization direction by 45°. In this way, and as shown in FIG. 4d,the radial polarization of the exiting light extends over the entirecross section.

As in the embodiment of FIGS. 1a and 1 b, this embodiment affords theadvantage of being especially thin and, as shown in the embodiment ofFIG. 2, has the advantage that precise radial polarization is providedwithout complex assembly of many facets or segments. The main advantageis also the high efficiency because the polarization is rotated and notselected. If, in lieu of an annular beam 41, a complete beam istransmitted through the arrangement, then the core area is simply notinfluenced.

FIG. 5 is a schematic showing a complete microlithographic projectionexposure system with a radially polarizing optical arrangement 55 whichis here in the form of a conical-frustrum polarizer according to FIG. 2.Except for this element and its mounting, all components and theirarrangement are conventional. A light source 51, for example, an i-linemercury discharge lamp having mirror 52, illuminates a diaphragm 53. Thei-line mercury lamp is tuned to the i-line (atomic emission spectralline of mercury having a wavelength of 358 nm) and is conventionallyused in microlithography. An objective 54 (for example, a zoom axiconobjective as disclosed in German patent publication 4,421,053) followsand makes possible various adjustments, especially the selection of anannular aperture.

The conical-frustrum polarizer 55, which is suitable for unpolarizedentering light, is followed by: a honeycomb condenser 56 and a relay andfield optic 57. These parts together serve to optimize illumination ofthe reticle 58 (the mask) which is imaged by the projection objective 59at a reduced scale and with the highest resolution (below 1 μm) on theresist film 60 of the wafer 61. The numerical aperture of the systemlies in the range of values above 0.5 to 0.9. Annular apertures between0.7 and 0.9 are preferred. The radial polarization of the light afterleaving the conical-frustrum polarizer 55 causes the effect of thestress-induced birefringence to be rotationally symmetrical with respectto the optical axis at all of the following optical elements (56, 57,58, 59). The effect is the greatest at the entrance into the resist film60 where the largest inlet angles occur and therefore optimaltransmission and minimum reflection are achieved. The sensitive beampath in the projection objective 59 is undisturbed.

The embodiment of the polarizing optical arrangement 55 is not limitedto the embodiment of FIG. 2. Especially all polarization-rotatingarrangements can be used and, if needed, a polarizer or birefringentplate can be mounted forward of the arrangement for adaptation. Also, apolarization-rotating optical arrangement 55 can be placed at otherlocations in the overall configuration. As noted above, allpolarization-rotating arrangements can be used and FIG. 7 shows anembodiment of a microlithographic projection exposure systemincorporating the radially polarization-rotating optical arrangement 62shown in detail in FIGS. 1a and 1 b.

This is especially true when deflection mirrors without phase correctionor polarizing elements, such as polarization beam splitters, are used.Then, the polarization-rotating optical arrangement according to theinvention is placed behind these elements as viewed in light flowdirection. One embodiment is shown in FIG. 6 in the context of acatadioptric projection objective.

FIG. 6 corresponds completely to FIG. 1 of European patent publication0,602,923 having polarizing beam splitter 103, concave mirror 106, lensgroups (102, 105, 108) and quarter-wave plate 104. Thepolarization-rotating optical element 107 is, however, not aquarter-wave plate for circular polarization and therefore uniformdeterioration of the coupling in of light into the resist 109, asdescribed initially herein with respect to European patent publication0,602,923. The polarization-rotating optical element 107 also is not ameans for aligning the uniform linear polarization to a preferreddirection of the pattern on the reticle 101. Rather, a radialpolarization-rotating optical arrangement 107 is provided in FIG. 6.

The embodiments of FIGS. 1a and 1 b and 4 a are the best suited herebecause of the small amount of space available. The advantage is clear,namely, independently of the pattern of the individual case, optimalscatter light suppression and uniform efficiency of the incoupling oflight into the resist 109 is achieved.

The radial polarizing optical arrangement 107 is mounted as close aspossible behind the deflecting mirror 103 a in the almost completelycollimated beam path, that is, in a range of moderate angles anddivergences of the light rays. Small angles are important for atrouble-free functioning of the birefringent elements. The best effectis achieved when the exit plane of the polarization-rotating elementslies in a plane of the illumination or projection system which isfourier-transformed to the image plane or in a plane equivalent thereto.

The use of the polarization-rotating optical arrangement, whichgenerates a radially orientated linear polarization on the total beamcross section, is not limited to microlithography.

It is understood that the foregoing description is that of the preferredembodiments of the invention and that various changes and modificationsmay be made thereto without departing from the spirit and scope of theinvention as defined in the appended claims.

What is claimed is:
 1. A microlithographic projection exposure systemcomprising: an illumination system including a light source and havingan optical axis featuring a light bundle which has a cross section wherepolarization of light is essentially orientated radially everywhere inthe cross section; an optical structure including a frame and more thanfour half-wave plates disposed in a facet configuration; said half-waveplates having respective preferred directions so arranged that eachhalf-wave plate deflects the polarization direction of the polarizedlight in the direction of a radius which cuts through the correspondinghalf-wave plate and is directed to said optical axis; said systemfurther including a honeycomb condenser operatively connected to saidoptical structure so as to cause each honeycomb of said honeycombcondenser to be assigned to a facet of said facet configuration; areticle arranged on said optical axis downstream of said opticalstructure thereby placing said optical structure between said lightsource and said reticle; and, a projection lens mounted downstream ofsaid reticle.
 2. A microlithographic projection exposure systemcomprising: an illumination system including a light source and havingan optical axis featuring a light bundle which has a cross section wherepolarization of light is essentially orientated radially everywhere inthe cross section; more than four half-wave plates disposed in a facetconfiguration; said half-wave plates having respective preferreddirections so arranged that each half-wave plate deflects thepolarization direction of the polarized light in the direction of aradius which cuts through the corresponding half-wave plate and isdirected to said optical axis; said system further including a honeycombcondenser operatively connected to said half-wave plates so as to causeeach honeycomb of said honeycomb condenser to be assigned to a facet ofsaid facet configuration; a reticle arranged on said optical axisdownstream of said half-wave plates thereby placing said half-waveplates between said light source and said reticle; and, a projectionlens mounted downstream of said reticle.